(1) Field of the Invention
The present invention relates to wafer cleaning methods and apparatus for etching, and dissolving and removing metallic impurities from various types of wafers such as semiconductor substrates, glass substrates for photomasks and glass substrates for liquid crystal displays.
(2) Description of the Prior Art
Japanese Patent Publication Laying-Open No. 173720/1987 discloses such a cleaning technique, which comprises the steps of (1) placing a wafer in a treating chamber, (2) supplying vapor of hydrofluoric acid HF/H.sub.2 O into the treating chamber to dissolve and wash oxide films off a wafer surface, (3) stopping the supply of hydrofluoric acid vapor after completion of oxide film removal and supplying high purity water vapor into the treating chamber to wash off hydrofluoric acid adhering to the wafer surface and inside walls of the treating chamber, and (4) stopping the supply of water vapor after hydrofluoric acid is sufficiently replaced by water, and supplying hot nitrogen gas N.sub.2 into the treating chamber to dry the wafer.
As methods of producing vapor of hydrofluoric acid from a tank storing a hydrofluoric acid solution, the above publication discloses (a) heating of the tank, (b) injection of nitrogen gas into the solution to cause its bubbling, and (c) use of an ultrasonic generator.
Further, according to this known technique, a wafer supporting device is mounted in a housing, with the tank disposed at a lower lateral position of the housing for storing hydrofluoric acid acting as cleaning liquid. The tank and housing communicate with each other through a pipe including a valve. Hydrofluoric acid vapor is generated as noted above, which is supplied into the housing to dissolve and wash oxide films off the wafer surface.
Japanese Patent Publication Laying-Open No. 213127/1987 discloses a different cleaning technique. This technique is carried out by (1) supplying clean nitrogen gas N.sub.2 into a treating chamber while spinning a wafer placed in the treating chamber, (2) in parallel with the above, supplying hydrogen fluoride gas HF, which is obtained by heating anhydrous hydrofluoric acid, and spraying superpure water into the treating chamber at the same time to produce hydrofluoric acid for removing oxide films from the wafer, (3) cleaning the wafer surface with jets of superpure water, and (4) spinning the wafer rapidly while introducing nitrogen gas to extract the liquid from and dry the wafer.
In both of the techniques disclosed in Patent Publications Laying-Open Nos. 173720/1987 and 213127/1987, the wafer before being dried is washed by supplying highly pure water vapor to the wafer surface and chamber walls or by spraying superpure water against the wafer. These techniques, however, have the drawback of leaving particles on the wafer surface in spite of the washing treatment.
In the technique disclosed in Patent Publication Laying-Open No. 173720/1987, an aerosol (mist) is formed along with the vapor by any one of the methods (a)-(c) of producing hydrofluoric acid vapor. That is, an aerosol is formed by boiling in the case of method (a), by formation of bubbles in method (b), and by cavitation in method (c).
Moreover, a space must be secured outside the housing to install the tank for storing hydrofluoric acid. This results in the disadvantage of enlarging the entire apparatus.
With such a wafer cleaning apparatus, it is necessary to prevent leakage of the vapor such as of hydrofluoric acid used in the cleaning treatment. Thus, seals must be provided not only for the housing but for the tank itself and connections of the pipe to the tank and housing. Such sealing structures are large and expensive.
In the technique disclosed in Patent Publication Laying-Open No. 213127/1987, the supply of superpure water and the direct supply to the wafer of the liquid hydrofluoric acid formed by dissolving hydrogen fluoride gas in the superpure water themselves result in adhesion of an aerosol to the wafer.